The invention generally relates to a turbine blade arrangement with moving blades.
To increase the efficiency or the turbine power output and therefore the effective cross section of turbines, conventionally the blade profiles of the turbine moving blades are lengthened, in order thereby to achieve a better utilization of the hot working fluid flowing past or more power output. However, this lengthening of the blade profile is limited by several parameters.
In particular, the lengthened blade profiles and the consequently increased moved mass exert a high load on a hub region of the turbine disk due to the centrifugal force which is applied. Attempts are made to counteract this by increasing the carrying surface in the hub region by way of an axial lengthening of the disk. However, this lengthening possibility is limited. Enlarged blade profiles not only subject the hub to higher load, but also the region in which the turbine blades are inserted with their roots into grooves of the outer circumference of the turbine disk. A lengthening of the blade profiles could also take place in the direction of the disk hub. As a result of this, however, the distance between the grooves of the outer circumference would become smaller and therefore the disk region between them, and, in particular, the groove regions nearest the hub, designated as the root cut, would be subjected to even greater load. At the present time, however, this load is virtually at its maximum possible and can almost no longer be increased, without risking damage to the turbine disk.
An object of an embodiment of the present invention is, therefore, to provide a turbine blade arrangement which makes it possible to lengthen the moving blade profiles, without an increase or with a merely insignificant increase in the local loads on grooves of the turbine disk or on moving blade roots.
The object may be achieved in at least a part of the platform is connected to the turbine disk by way of a holding device independent of the blade root. By the platform being connected to the turbine disk, at least some of the centrifugal force load caused by the moving blades rotating together with the turbine disk is transferred by the holding device to the turbine disk, to regions located between the root regions. At least some of the centrifugal force load therefore does not have to be absorbed by the blade root or the groove into which the root is inserted and does not have to be transferred to the turbine disk. As a result of load redistribution, therefore, the load is introduced more uniformly into the turbine disk, and the roots of the moving blades and the grooves into which the roots are pushed are relieved of stress excesses which are detrimental to the strength of the regions. This is important particularly in the region of the root cut, imagined as a circle around the hub and running through the lowest groove regions, since the highest stress excesses occur in the lowest groove regions.
Moreover, it is possible for transitional regions between platform and blade to be made appreciably less thick and massive, since the lever forces occurring in this region in a conventional blade due to a projecting platform fastened to the blade are absorbed completely owing to the use of the holding device. The narrow design results, in addition, in a further weight saving. The profiles of the moving blades can therefore be lengthened without an increase or, depending on the amount of lengthening, with a merely insignificant increase in the local loads on disk grooves or moving blade roots. Thus, the efficiency of the turbine can be increased without any adverse influence on the strength of the disk and of the blades.
If the platform part connected to the turbine disk by way of the holding device is produced separately from the moving blade, the holding device absorbs the entire centrifugal force load caused by the platform part. The groove is therefore no longer subjected to load. By the masses of the platform part and of the moving blades, with blade profile and blade root, being separated completely, the centrifugal forces which take effect are absorbed separately by virtue of the respective connection to the turbine disk. The holding device and root therefore have to transfer in each case only a relatively small part of the total centrifugal force load. In the region in which the platform part is separated from the blade, that is to say at edges, it is possible to have a less massive construction of the blade and platform part than in the case of a one-part blade not additionally connected to the turbine disk, since the weight of the platform does not also have to be carried in addition. In this way, therefore, the total weight of the blade is reduced, on the one hand, by the platform being separated and, moreover, by the less massive construction at the edges. The root and the groove thus have to carry even less weight. Moreover, the blades, with the blade profiles, and the separately fastened platform parts are not so easily set in vibration which is critical for the blade fastening, or the vibrations can be damped more easily than in the case of the one-part construction of the blade. Furthermore, the blade and the platform part can be produced separately at a substantially lower outlay. In particular, where the casting of the blade is concerned, the production of the casting mold and exact casting execution are simplified, since the turbine blade without the integrally formed platform virtually no longer has any projecting integral part. The isolated platform part has a simple geometric shape, in general is plate-shaped, and can therefore be produced at a low outlay. Moreover, different materials can be used for the blade and the platform part. As a result, if a relatively light alloy is used, weight and, if appropriate, material costs and machining costs can be saved.
A uniform distribution of the acting centrifugal forces over the circumference of the turbine disk is achieved in that a one-piece platform part is used as a platform part of two adjacent moving blades and the holding device is arranged approximately in the middle between the two adjacent moving blades. The stress peaks, which occur, in particular, below the lowest toothing of the groove due to the high centrifugal force load, are thereby greatly reduced. Since a one-piece platform part is connected to the turbine disk between two moving blades, the number of required platform parts and holding devices for the platform parts is lowered respectively to one platform part and one holding device between two adjacent moving blades in each case.
The largest possible surface fraction of the platform part is achieved in that the platform part is inserted between the end regions of two adjacent blade profiles in such a way that it replaces the platforms virtually completely. Almost the entire platform masses are therefore carried by the holding device and do not exert load either on the roots or on the grooves into which the roots are pushed. An optimum mass distribution to the root and the holding device is thus achieved. In the separation regions in which the platform part and the blade are adjacent, a large amount of material and therefore weight is saved, as compared with a one-part construction, since it is no longer necessary to absorb the lever forces occurring due to the large platform part. A great material saving is also made possible by the fact that the edges of the platform part which are adjacent to the blade profiles are shaped in adaptation to the curvature of the blade profiles.
Moreover, production is simplified, since, in this case, the blade has a slender shape, even in the transitional region between the root and profile, this shape being substantially simpler to cast. Stable and at the same time flexible adaption of the holding device to the platform part and to the turbine disk is afforded in that the holding device consists of at least one pairing of holding partners engaging one into the other, at least one connection element which has one holding partner being formed separately from the platform part and from the turbine disk. By the holding partner being formed separately, the platform part can be attached to the turbine disk by way of various methods and so as to be easily exchangeable. Furthermore, various material combinations between the parts are thereby possible.
In particular, the material of a separately formed platform part and of the separately formed holding partner, and also of the turbine disk and the blade, may be different and be selected in a cost-optimized manner, taking into account the respective requirements and loads.
If the holding partner is connected to the turbine disk and to the platform part by form-locking which withstand centrifugal force loads, a holding device of this type can easily be released, for example for repair purposes, and can be reused afterward without any restriction in its functioning.
The holding device can easily be installed and also, in the event of a possible corrosion attack, be removed again at a low outlay when the holding partner is connected with play to the platform part and to the turbine disk. At the same time, in the event of forces being applied from different directions or of a sharply alternating force, the holding device is better suited to reacting flexibly and to being set more easily in the corresponding force direction, with the result that damage to the holding device and the form-locking device connected to it and also to the platform part and the turbine disk is avoided.
Simple attachment is afforded in that one holding partner runs rectilinearly over a coupling length and has a rail-like cross section, and the other holding partner of the pairing runs rectilinearly parallel to the first holding partner and has a cross section surrounding the rail-like cross section of the first holding partner in a form-locking manner. The rail-like construction of the holding partners over the entire coupling length provides large bearing and contact surfaces and therefore good force distribution over the entire region of the coupling. Local stress peaks as a result of the centrifugal forces which act are thus reduced. Particularly in the case of the curved construction of the platform part, the platform part is seated on the turbine disk in a highly reliable way by use of the rail-like holding partners.
A secure hold is afforded when one rail-like holding partner is connected to the platform part and one rail-like holding partner is connected to the turbine disk and both holding partners are connected to two holding partners by way of a connection element, the connection element having an H-shaped cross section surrounding the rail-shaped cross sections in a form-locking manner. The holding partners are connected to one another in a form-locking manner over a large region. The connection is simple to make and can easily be released again. By virtue of the rail-like constructions of the holding partners, the connection element having the H-shaped cross section can easily be pushed in and pulled out between the platform part and the turbine disk. Since no holding partner has a complex shape, they can be produced at a low outlay and cost-effectively.
A highly stable holding device is provided when it is constructed in such a way that the turbine disk has a rail-like holding partner and the platform part has a rail-surrounding holding partner and both are connected by way of a connection element which has a rail-surrounding holding partner and a rail-like holding partner.